1. Field of the Invention
The present invention relates generally to an electrophotographic (EP) image forming device and, more particularly, to a system and method for adjusting a selected operating parameter, namely bias voltage of the charge roll, of the image forming device based on selected environmental conditions, namely, wet-bulb temperature values derived from dry-bulb temperature and relative humidity sensor readings, to control white vector, the difference in surface potential of non-discharged areas of a photoconductive (PC) drum from surface potential of a developer roll, and thereby reduce background toner.
2. Description of the Related Art
The electrophotography (EP) process used in image forming devices, such as laser printers and copiers, utilizes electrical potentials between components to control the transfer and placement of toner. These electrical potentials create attractive and repulsive forces that tend to promote the transfer of charged toner to desired areas while ideally preventing transfer of the toner to unwanted areas. For instance, during the process of developing a latent image on the surface of a PC drum, negatively charged toner particles deposit onto less negatively charged (positive relative to the toner's charge) latent image feature areas (e.g., corresponding to text or graphics) on the PC drum surface. At the same time, the negatively charged toner particles are prevented from transferring or migrating to more negatively charged areas (e.g., corresponding to the document background) of the same PC drum surface. In this manner, image forming devices implementing this process can simultaneously generate images with fine detail while maintaining clean backgrounds.
In general, a laser imaging source is used to illuminate and selectively discharge the desired areas of the PC drum surface to create the latent image so that it will have a lower surface potential than the remaining, undischarged areas of the PC drum surface. The developer roll, where a layer of charged toner is located, is biased to an intermediate level between the discharge potential of the latent image and the surface potential of the undischarged areas of the PC drum surface. The toner can be charged triboelectrically and/or via biased roll delivery control components, such as a toner adder roll, a doctor blade, and a developer roll. The developer roll supplies toner to develop the latent image areas on the PC drum surface. The developed image is ultimately transferred onto a media sheet, typically by employing yet another surface potential that attracts the toner off of the PC drum surface (or an intermediate transfer surface) and onto the media sheet where it is ultimately fused.
The difference between the surface potential of the developer roll and the surface potential of undischarged areas of the PC drum surface is referred to as a white vector. An optimal white vector achieves certain desirable characteristics, one of which is to provide a clean media sheet with little or no appreciable background toner in areas other than where printing is desired. The magnitude of the white vector needed to prevent background toner is a function of numerous factors, including developer material, environment, imaging device components, and age. Traditionally, image forming devices incorporating an EP process operate with a white vector that is fixed, but large enough to overcome the factors that contribute to unwanted background toner.
Very large white vector values are not necessarily the most desirable solution because the density of deposited toner and detail of the resulting image may be adversely affected and could lead to background toner if wrong-sign toner (toner charged positively) is present. Conversely, as white vector values fall, unwanted background toner may begin to appear. Determining an optimal white vector that is somewhere between these extremes and that accounts for the aforementioned factors and varying operating conditions is a legitimate problem that is not solved by setting a fixed operating point.
Although a majority of image forming devices, such as laser printers, operates in an air-conditioned office environment, such environment may not necessarily be controlled for humidity. It is important that a printer yields high print quality over a wide range of environments. As temperature and humidity of the ambient environment change, the electrical properties of printer components can also change which can have a significant impact on print quality. Heretofore, “cold start” servo voltage has been used to select or adjust charge roll bias. Cold start servo voltages are the servo values recorded when the printer is first powered on or after the printer has been idle. However, print quality requirements have made servo algorithms not accurate enough for optimizing charge roll bias to minimize background toner in all environments.
One approach to resolving this problem of controlling white vector is disclosed in U.S. Pat. No. 7,398,025 assigned to the assignee of the present application. The entire disclosure of this patent is hereby incorporated herein by reference. This patent proposes to control and adjust white vector by using one or more control circuits adapted to control the formation of a given image pattern on a substrate, such as a transport belt, transfer belt, or media sheet. The circuits utilize sensors to detect the coverage of the developed image pattern on the PC drum surface or on the substrate. White vector may then be adjusted in response to a comparison between the detected coverage of the developed image and the desired coverage of the developed image. In one embodiment, background noise is used as an indicator that white vector needs to be adjusted. In another embodiment, reflectance of a developed pattern is used to detect the coverage or bloom of the pattern relative to a predetermined standard. Iterative procedures also are used to determine an optimum operating point.
While the approach of this patent might represent a step in the right direction toward resolution of this problem, its implementation is not always feasible requiring an innovation that will provide an alternative approach to its resolution.